Last year my mother's beloved kitty cat (just two years old) began suffering from a faulty metabolism that caused everything he ate to turn into sand in his bladder. She had him cleaned out, even paid for a very expensive operation to reroute his "plumbing" (an operation that works in most cases). But it didn't work for him. His metabolism simply quit on him, and nothing could help. Even after the operation his bladder filled up till it lost its elasticity. He'd probably have required periodic surgery just to stay alive (which she couldn't have afforded). But she never had to make that decision because he died on the operating table. I can't tell you how hurt she was or how it hurt me to see her suffer and grieve so.
These are things you simply don't get over. We know that for people there is a World to Come. I don't know if there is one for our beloved pets or not, which makes losing them so traumatizing (I'd like to think they'll be there, but I must defer this decision to our beloved Creator).
I am so sorry for what you are going through. This isn't going to help any, but I just want you to know that this happens to everyone who has an animal friend. It's inevitable. You are not alone in your pain.
May G-d comfort you.
You have absolutely no idea how much that means to me. It does feel lonely and knowing you and so many others are here and have had to go through the same is very comforting.
Thank you for your kind words.